1. Field of the Invention
The present invention relates to a technology for decoding an audio signal.
2. Description of the Related Art
Recently, the High-Efficiency Advanced Audio Coding (HE-AAC) method is used for encoding voice, sound, and music. The HE-AAC method is an audio compression method, which is principally used, for example, by the Moving Picture Experts Group phase 2 (MPEG-2), or the Moving Picture Experts Group phase 4 (MPEG-4).
According to encoding by the HE-AAC method, a low-frequency component of an audio signal to be encoded (a signal related to such as voice, sound, and music) is encoded by the Advanced Audio Coding (AAC) method, and a high-frequency component of the audio signal is encoded by the Spectral Band Replication (SBR) method. According to the SBR method, the high-frequency component of the audio signal can be encoded with bit counts fewer than usual by encoding only a portion that cannot be estimated from a low-frequency component of the audio signal. Hereinafter, data encoded by the AAC method is referred to as AAC data, and data encoded by the SBR method is referred to as SBR data.
According to the encoding by the HE-AAC method, the higher the frequency band, the wider the bandwidth divided. Power of the audio signal is evened out in a divided band, and then the audio signal is encoded. As shown in FIG. 15, the audio signal is encoded according to the encoding by the HE-AAC method for the higher the frequency (the frequency of the high-frequency component to be encoded by the SBR method), to the wider the bandwidth divided.
An example of a decoder for decoding data encoded by the HE-AAC method (HE-AAC data) is explained below. As shown in FIG. 16, the decoder 10 includes a data separating unit 11, an AAC decoding unit 12, an analyzing filter 13, a high-frequency creating unit 14, and a synthesizing filter 15.
When the data separating unit 11 acquires the HE-AAC data, the data separating unit 11 separates the HE-AAC data into the AAC data and the SBR data, outputs the AAC data to the AAC decoding unit 12, and outputs the SBR data to the high-frequency creating unit 14.
The AAC decoding unit 12 decodes the AAC data, and outputs the decoded AAC data to the analyzing filter 13 as AAC decoded audio data. The analyzing filter 13 calculates characteristics of time and frequencies related to the low-frequency component of the audio signal based on the AAC decoded audio data acquired from the AAC decoding unit 12, and outputs the calculation result to the synthesizing filter 15 and the high-frequency creating unit 14. Hereinafter, the calculation result output from the analyzing filter 13 is referred to as low-frequency component data.
The high-frequency creating unit 14 creates a high-frequency component of the audio signal based on the SBR data acquired from the data separating unit 11, and the low-frequency component data acquired from the analyzing filter 13. The high-frequency creating unit 14 then outputs the created data of the high-frequency component as a high-frequency component data to the synthesizing filter 15.
The synthesizing filter 15 synthesizes the low-frequency component data acquired from the analyzing filter 13 and the high-frequency component data acquired from the high-frequency creating unit 14, and outputs the synthesized data as HE-AAC output audio data.
Processing performed by the decoder 10 is explained below. The analyzing filter 13 creates low-frequency component data as shown in the left part of FIG. 17. As shown in the right part of FIG. 17, the high-frequency creating unit 14 creates high-frequency component data from the low-frequency component data, and the synthesizing filter 15 synthesizes the low-frequency component data and the high-frequency component data to output the HE-AAC output audio data. Thus, the decoder 10 decodes the audio signal encoded by the HE-AAC data method into the HE-AAC output audio data.
Japanese Patent Application Laid-open No. 2002-73088 discloses a technology for accurately restoring a signal, even if a high-frequency portion of the signal is steeply attenuated. According to the technology, spectra are divided into bands; frequency bands having a strong correlation between each other combined into a pair for deletion and interpolation; the bands for deletion are eliminated and the rest of the bands is shifted to the lower frequency side; and a signal in the higher frequency side is saved; so that the audio signal is compressed while retaining a high sound quality.
However, the conventional technology described above has a problem that the high-frequency component of the audio signal encoded by the SBR method cannot be properly decoded due to poor frequency resolution for the audio signal encoded by the SBR method.
Under the conventional SBR method, the bandwidth of a band to be encoded is wide (the frequency resolution of the SBR method is poor). As shown in FIG. 18, if a portion of a sound, such as a consonant, in which power steeply drops in a band on the high-frequency component side, is encoded with a wide bandwidth, the power within the band is evened out, so that the power is even between the low-frequency side and the high-frequency side, consequently the high-frequency side within the band is emphasized.
As shown in FIG. 18, the audio signal is encoded in a state where the high-frequency side within the band is emphasized. If the audio signal is decoded based on such encoded audio signal, the encoded audio signal is decoded as the high-frequency side within the band is emphasized, so that the audio signal cannot be properly decoded.
In other words, it is strongly required that a decoded audio signal is accurately decoded by compensating the high-frequency component, even if the high-frequency component of the audio signal is not properly encoded.